Do you think it's a good idea for a 79 year old woman?

i don't think she'll be doing the real raising of them...there'll be others to run around them & do the practical things. Shes there as their guardian, their connection to their Dad and they are her responsibility. Grandparents the world over fulfill that role beautifully...i truly believe there is no connection like the one Grandparents share with their Grandchildren. Yes, all too soon, they will lose her but they will have a foundation of love & caring that you say she has for them, which hopefully will stand them in good stead.